diff --git a/Python/marshal.c b/Python/marshal.c
index 04a8dc5..4a23df1 100644
--- a/Python/marshal.c
+++ b/Python/marshal.c
@@ -734,7 +734,7 @@
     else {
         const char *ptr = r_string(1, p);
         if (ptr != NULL)
-            c = *(unsigned char *) ptr;
+            c = *(const unsigned char *) ptr;
     }
     return c;
 }